1. Field of the Invention
The present invention relates to a vehicle-installed exhaust gas analyzing apparatus capable of real time measuring concentration of hydrocarbons (HC) contained in exhaust gas emitted from an engine of a vehicle such as automobile running on the road. The present invention also relates to a vehicle-installed exhaust gas analyzing apparatus for continuously measuring mass of total hydrocarbon (THC) (mass emission) contained in exhaust gas from an engine of a vehicle such as automobile. Furthermore, the present invention relates to a vehicle-installed exhaust gas analyzing apparatus equipped with a differential pressure type flowmeter such as Pitot tube type flowmeter.
2. Background Art
In recent years, increasing concerns about influence of exhaust gas emitted from engines etc. (hereinafter, simply referred to as exhaust gas) on the environment have raised the trend that the actual condition should be grasped in a condition similar to the actual environment rather than in laboratory or test course. Researches and developments (seeking) for procedures of measuring components emitted from a vehicle running on the road such as nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2) and the like have been carried out. Also in the trend, it is requested to measure not only concentration of each component, but also mass emission thereof. One example that satisfies the above requirement is a vehicle-installed engine exhaust gas analyzer described in Japanese Unexamined Patent Publication 2001-124674. According to this vehicle-installed engine exhaust gas analyzer, it is possible to continuously and simultaneously measure a plurality of components such as CO, CO2, NO, N2O, H2O, NH3 and HCHO contained in exhaust gas emitted from a vehicle on the actual road. Furthermore, the above vehicle-installed engine exhaust gas analyzer is equipped therein with an exhaust gas concentration analyzing device for analyzing component concentration in exhaust gas, and an exhaust gas flow rate measuring device wherein a predetermined amount of known concentration of trace gas is injected from the upstream side of the measurement point by this exhaust gas concentration analyzing device, and the flow rate of the exhaust gas is obtained from the injection concentration and injection flow rate of the trace gas injected into the exhaust gas, whereby mass emission of the components contained in the exhaust gas is determined based on the component concentration obtained by the exhaust gas concentration analyzing device and the exhaust gas flow rate obtained by the exhaust gas flow rate measuring device.
By the way, in the field of exhaust gas analysis, requests for real-time quantitative analysis of HC have been increased in recent years. In the aforementioned vehicle-installed engine exhaust gas analyzer, however, since a Fourier transform infrared analyzer (FTIR) is used for component analysis, measurement of THC is difficult, the equipment is bulky and expensive, so that it is not suited for use on board. On the other hand, as an apparatus for measuring THC concentration, a method that uses a flame ionization detector (FID) is known. Since this FID method provides excellent stability and response in proportion to the number of contained carbons, it has been conventionally used for analyzing THC concentration contained in atmospheric air or in combustion gas emitted from a combustion apparatus such as boiler.
In the above FID method, however, an operation gas such as fuel hydrogen or supporting air is required for analysis, so that there is a problem that it is not suited for vehicle-installed application wherein compact size and measurement with simple operation are requested.
The present invention was devised in consideration of these facts, and it is an object of the present invention to provide a vehicle-installed exhaust gas analyzing apparatus capable of readily measuring THC concentration and mass in the exhaust gas in a vehicle running on an actual road.
Furthermore, according to the aforementioned vehicle-installed engine exhaust gas analyzing apparatus, though it is possible to continuously measure concentration and mass of a specific component contained in exhaust gas, it is necessary to mount a trace gas source in the vehicle and inject trace gas into the exhaust pipe while measuring the trace gas from an appropriate position for measuring exhaust gas flow rate by means of the exhaust gas flow rate measuring device, so that configuration for measuring exhaust gas flow rate is complicated. Furthermore, when the exhaust gas flow rate measuring device is used in combination with the exhaust gas concentration analyzing device, problems of deviation in time axis and difference of response between these devices arise, so that an accurate flow rate is not usually obtained. Therefore, it is impossible to accurately determine mass of every component to be measured contained in the exhaust gas.
The present invention was devised in consideration of these facts, and it is an object of the present invention to provide a vehicle-installed exhaust gas analyzing apparatus capable of continuously measuring THC mass contained in the exhaust gas in a vehicle running on an actual road with simple configuration and with high accuracy.
As one example of differential pressure type flowmeter for measuring flow rate of gas flowing through a pipe, a Pitot tube type flowmeter is well known. In a Pitot tube type flowmeter, gas flow rate (in terms of normal state) Qg (t) [m3/min] can be represented by the following formula (7):                                           Q            g                    ⁡                      (            t            )                          =                  K          ×                                                                                          P                    g                                    ⁡                                      (                    t                    )                                                  101.3                            ×                              293.15                                                      T                    g                                    ⁡                                      (                    t                    )                                                              ×                                                Δ                  ⁢                                                                           ⁢                                      h                    ⁡                                          (                      t                      )                                                                                        γ                  g                                                                                        (        7        )                            (wherein K: proportion coefficient        Pg (t): gas pressure [kPa]        Tg (t): gas temperature [° K]        Δh (t): differential pressure of Pitot tube        γg: gas density in normal state [g/m3]        
That is, by determining the proportion coefficient K in advance, it is possible to obtain flow rate of the gas from the temperature and pressure of the gas flowing through the pipe and measurement of differential pressure of the Pitot tube.
By the way, since a Pitot tube type flowmeter utilizes the fact that flow rate and square root of differential pressure has a proportional relationship as can be seen from the above formula (7), a differential manometer having wide range is required for measuring the dynamically-varying flow rate. For example, a range of 10000 times is required with respect to a flow rate range of 100 times. Also, as is the case of exhaust gas from an automobile engine, when the flow rate rapidly varies dynamically, it is impossible to average the data. Furthermore, when the engine is in idle state, the differential manometer is influenced by pulsation of the exhaust gas even though the flow rate is low, making measurement extremely difficult. In such a case, there arises another problem that response speed in high flow rate zone is decreased in compensation for attempt to ensure measurement accuracy in low flow rate zone.
For this reason, conventionally, dynamic range was often measured by using several kinds of flowmeters of different ranges as described in Japanese Unexamined Patent Publication 2001-41787, for example. However, in such a manner, a space for installing the plurality of flowmeters is required and the cost rises, and correlation among the plurality of flowmeters often raised a problem.
The present invention was devised in consideration of the facts as described above, and it is an object of the invention to provide a vehicle-installed exhaust gas analyzing apparatus including a differential pressure type flowmeter having a wide measurement range capable of securely reducing the noise in low flow rate zone without reducing the response speed in high flow rate zone, namely, capable of continuously measuring gas flow rate at desired response speed and with high accuracy from low flow rate zone to high flow rate zone.